ASME B16.5 is one of the most widely referenced piping component standards in industrial engineering. It covers flanges and flanged fittings from NPS ½ through NPS 24, defining seven pressure classes — 150, 300, 400, 600, 900, 1500, and 2500. Understanding these classes and what they mean in practice is essential for anyone specifying, purchasing, or designing with stainless steel or duplex flanges.
What is a Pressure Class?
A pressure class (also called a pressure-temperature rating) is a designation that defines the maximum allowable working pressure (MAWP) of a flange at a given temperature. It is NOT a fixed pressure — it is a temperature-dependent rating. The same Class 150 flange has a lower MAWP at 250°C than at ambient temperature (38°C).
The "class number" was historically derived from a pound-force per square inch (psi) rating at a reference temperature, which is why you will see it referred to as a "pound rating" (150#, 300#, 600#, etc.) in some specifications. This convention persists in ASME B16.5 though all current pressure-temperature tables are published in both psi and bar (MPa).
The Seven Pressure Classes
| Class | MAWP at 38°C for Group 2.2* (MPa / bar) | Typical Applications |
|---|---|---|
| 150 | 19.6 bar / 285 psi | Low-pressure utilities, water, steam below 200°C |
| 300 | 51.1 bar / 740 psi | General process piping, steam, oil & gas moderate pressure |
| 400 | 68.3 bar / 990 psi | Less common — used in some specialised applications |
| 600 | 102.1 bar / 1480 psi | High-pressure process piping, high-pressure steam |
| 900 | 153.4 bar / 2225 psi | Very high pressure; refinery, chemical plant, power generation |
| 1500 | 255.6 bar / 3705 psi | Wellheads, high-pressure gas, critical refinery service |
| 2500 | 425.3 bar / 6170 psi | Ultra high pressure; subsea wellheads, injection systems |
* Group 2.2 covers austenitic stainless steels such as 304, 304L, 316, and 316L under ASME B16.5. Duplex grades (S31803/S32205) fall in Group 2.3 and have different pressure-temperature tables with generally higher allowable pressures due to duplex's superior strength.
Temperature Effect on Pressure Rating
The pressure rating decreases as temperature increases because the allowable stress of the flange material decreases with temperature. For austenitic stainless steel (Group 2.2) at Class 300:
| Temperature | Class 300 MAWP (approx. for 316L) |
|---|---|
| -29°C to 38°C | 51.1 bar (740 psi) |
| 100°C | 46.6 bar (676 psi) |
| 200°C | 39.8 bar (577 psi) |
| 300°C | 34.1 bar (495 psi) |
| 400°C | 28.8 bar (418 psi) |
Always specify both design pressure and design temperature when selecting a flange class. Using only the pressure without the temperature is the most common specification error.
Material Groups Under ASME B16.5
ASME B16.5 groups flange materials by their temperature-strength characteristics. For stainless and alloy steels, the key groups are:
- Group 2.1: Carbon steel flanges (e.g., ASTM A105)
- Group 2.2: Austenitic stainless steels — SS 304, 304L, 316, 316L (ASTM A182 grades F304, F304L, F316, F316L)
- Group 2.3: Duplex and super duplex stainless steels — 2205, 2507 (ASTM A182 grades F51, F53, F55)
- Group 3.1: Nickel alloys — Monel 400, Inconel 600
- Group 3.2: Higher nickel alloys — Inconel 625, Hastelloy C276
Each group has its own pressure-temperature rating table in ASME B16.5. A Class 300 flange in duplex 2205 (Group 2.3) has a higher MAWP than a Class 300 flange in 316L (Group 2.2) because of duplex's superior strength.
ASME B16.5 vs ASME B16.47
ASME B16.5 covers NPS ½ to NPS 24. For larger bore flanges (NPS 26 to NPS 60), ASME B16.47 applies. B16.47 Series A is based on MSS SP-44 (heavy pattern) and Series B is based on API 605 (lighter pattern). When specifying large-bore flanges, confirm which series applies — they have different bolt circle diameters and are not interchangeable.
Marking and Traceability
ASME B16.5 requires flanges to be marked with: Manufacturer's name or trademark, material grade, class (e.g., CL300), size, pressure rating, and heat number (or equivalent traceability mark). Verify these markings match the mill test certificate before installation.
Flange Types Available Under ASME B16.5
ASME B16.5 covers the following flange facing and connection types, all available in the seven pressure classes:
- Weld Neck (WN) — most robust, preferred for high-pressure and cyclic service
- Slip-On (SO) — economical, for moderate service
- Blind (BL) — for pipe end closures
- Socket Weld (SW) — for small bore piping
- Threaded (TH) — for non-welded connections
- Lap Joint (LJ) — used with stub ends for ease of alignment
- Orifice — for flow measurement assemblies
- Ring Type Joint (RTJ) — for high-pressure, high-temperature service